The atomic mass of an atom is primarily determined by the combined mass of its protons, neutrons, and electrons, although electrons contribute very little to the overall atomic mass due to their significantly smaller mass compared to protons and neutrons.
Protons and neutrons, which are located in the nucleus of an atom, are collectively referred to as nucleons. Protons have a positive charge, while neutrons have no charge. Both protons and neutrons have approximately the same mass, with protons having a mass of approximately 1.67 × 10^(-27) kilograms and neutrons having a slightly larger mass of approximately 1.68 × 10^(-27) kilograms.
The sum of the protons and neutrons in an atom determines its mass number, which is the whole number found in the periodic table for each element. For example, carbon has an atomic mass of approximately 12 atomic mass units (AMU) because it typically has six protons and six neutrons, which sums up to a mass number of 12.
Electrons, which are negatively charged subatomic particles, have a much smaller mass compared to protons and neutrons. The mass of an electron is approximately 9.10938356 × 10^(-31) kilograms, which is about 1/1836th the mass of a proton or neutron. Since electrons have such a small mass, they contribute very little to the overall atomic mass of an atom.
In summary, the atomic mass of an atom is primarily determined by the combined mass of its protons and neutrons, with electrons contributing very minimally to the overall mass.